A mobile communication terminal represented by a cellular phone becomes increasingly popular around the world. A radio frequency circuit for use in the mobile communication terminal is disclosed in, for example, Japanese Patent Laid-Open No. H9-232887, Japanese Patent Laid-Open No. H10-56340 and Japanese Patent Laid-Open No. H11-112251.
An example of a radio frequency circuit disclosed in Japanese Patent Laid-Open No. H9-232887 will be described with reference to FIG. 1 of the publication. A radio frequency power amplifier shown in FIG. 1 includes an input matching network 10 that optimizes the impedance of an input signal and outputs a signal, a gallium arsenide power FET (field effect transistor) 21 that amplifies and outputs the signal that is outputted from the input matching network 10, a first output matching network 30 and a second output matching network 40 which optimize the impedance r L* of an output signal of the gallium arsenide power FET 21 according to a frequency band and outputs a signal, and a switch 27 that is connected between the gallium arsenide power FET 21 and the respective output matching networks 30 and 40. The switch 27 selects an optimum output matching network from the first and second output matching networks 30 and 40, and connects the gallium arsenide power FET 21 and the selected output matching network.
Another example of a radio frequency circuit disclosed in Japanese Patent Laid-Open No. H 10-56340 will be described with reference to FIG. 1 of that publication. A radio frequency amplifier shown in FIG. 1 includes a low-pass type input matching network 34 that matches two signals A1 (center frequency 902.5 MHz) and A2 (center frequency 1440 MHz), and a power amplifier circuit 37 that amplifies the transmitter input signals A1 and A2 that have been matched by the low-pass type input matching network 34 to a degree that can be used as the respective transmitter electric waves of a cellular phone and outputs its result as output signals B1 (center frequency 902.5 MHz) and B2 (center frequency 1440 MHz). In addition, the radio frequency amplifier includes a low-pass type output matching network 44 that is connected to an output side of the power amplifier circuit 37 and matches the transmitter output signal B1, and a high-pass type output matching network 49 that is connected in parallel with the low-pass type output matching network 44 at an output side of the power amplifier circuit 37 and matches the transmitter output signal B2. Then, the low-pass type output matching network 44 matches the transmitter output signal B1, and cuts off the transmitter output signal B2. Similarly, the high-pass output matching network 49 matches the transmitter output signal B2, and cuts off the transmitter output signal B1.
Subsequently, still another example of a radio frequency circuit disclosed in Japanese Patent Laid-Open No. H11-112251 will be described with reference to FIGS. 1 and 5 of this publication. A two-band radio frequency power amplifier shown in FIGS. 1 and 5 includes a stabilizer circuit A that is made up of a first resistor 101, a condenser 103, a third resistor 111, a line of quarter wavelength 112, and watersheds 113 and 114, a field effect transistor 104, input terminals 201 and 202, output terminals 203 and 204, and matching networks 211 to 214. In addition, the amplifier has a low-pass filter 402, an antenna switch 403 and an antenna 404 at an output side thereof. A signal of a first frequency band that is inputted from the input terminal 201 is amplified by the field effect transistor 104 and then outputted from the output terminal 203. An unnecessary harmonic distortion is suppressed from the signal by the low-pass filter 404, and the signal is transmitted from the antenna 404 through the antenna switch 403. Similarly, a signal of a second frequency band that is inputted from the input terminal 202 is magnified by the field effect transistor 104 and then outputted from the output terminal 204. An unwanted harmonic distortion is suppressed from the signal by the low-pass filter 402, and the signal is transmitted from the antenna 404 through the antenna switch 403.